This invention relates to vehicle stability, and more particularly, to a system and a method that enhance directional stability in locked drive systems.
Locked drive systems lock the front and the rear axles of a vehicle through a transfer case. The systems distribute torque to the front and the rear wheels, which improves traction and operator control in many circumstances. A locked drive system can include a locking differential that distributes a percentage of the engine and brake torque to the front and the rear axles. This distribution can enhance off-road performance, and in some instances, enhance driver control.
Sophisticated control systems, such as an Antilock Braking System (ABS) and a stability control system, are available on some locked drive systems. ABS modulates brake pressure during a hard braking maneuver to avoid wheel lock-up and a loss of steering control. A stability control system regulates engine torque and wheel braking when an operator loses a degree of control. While each of these systems is designed to provide maximum vehicle stability, the combination of the ABS, the stability control system, and a locked drive system can induce greater vehicle instability in some driving conditions.
In an over-steer event, for example, the ABS, the stability control overlay system, and the locked drive system can create vehicle instability by transferring front brake torque and engine torque to the rear axle. In the over-steer event, the rear of the vehicle slides, xe2x80x9cfishtailing,xe2x80x9d toward the outside of a turn, which causes the vehicle to yaw. As the rear wheels slide, the driver loses a degree of control. To avoid the spin, the stability control system brakes the outside front wheel. This brake pressure is designed to bring the rear wheels in alignment with the front wheels and provide a maximum stopping distance and an enhanced wheel control. In a locked drive system, however, a portion of this braking force combined with the engine torque is distributed to the rear axle. This additional torque slows down the rear wheels relative to the vehicle""s speed, which can increase tire slippage and cause even greater traction losses. Accordingly, there is a need for a locked drive system that can enhance vehicle stability under all driving conditions.